const std = @import("std.zig");
const builtin = @import("builtin");
const assert = std.debug.assert;
const testing = std.testing;
const os = std.os;
const math = std.math;

pub const epoch = @import("time/epoch.zig");

/// Spurious wakeups are possible and no precision of timing is guaranteed.
pub fn sleep(nanoseconds: u64) void {
    // TODO: opting out of async sleeping?
    if (std.io.is_async) {
        return std.event.Loop.instance.?.sleep(nanoseconds);
    }

    if (builtin.os.tag == .windows) {
        const big_ms_from_ns = nanoseconds / ns_per_ms;
        const ms = math.cast(os.windows.DWORD, big_ms_from_ns) orelse math.maxInt(os.windows.DWORD);
        os.windows.kernel32.Sleep(ms);
        return;
    }

    if (builtin.os.tag == .wasi) {
        const w = std.os.wasi;
        const userdata: w.userdata_t = 0x0123_45678;
        const clock = w.subscription_clock_t{
            .id = w.CLOCK.MONOTONIC,
            .timeout = nanoseconds,
            .precision = 0,
            .flags = 0,
        };
        const in = w.subscription_t{
            .userdata = userdata,
            .u = w.subscription_u_t{
                .tag = w.EVENTTYPE_CLOCK,
                .u = w.subscription_u_u_t{
                    .clock = clock,
                },
            },
        };

        var event: w.event_t = undefined;
        var nevents: usize = undefined;
        _ = w.poll_oneoff(&in, &event, 1, &nevents);
        return;
    }

    if (builtin.os.tag == .uefi) {
        const boot_services = os.uefi.system_table.boot_services.?;
        const us_from_ns = nanoseconds / ns_per_us;
        const us = math.cast(usize, us_from_ns) orelse math.maxInt(usize);
        _ = boot_services.stall(us);
        return;
    }

    const s = nanoseconds / ns_per_s;
    const ns = nanoseconds % ns_per_s;
    std.os.nanosleep(s, ns);
}

test "sleep" {
    sleep(1);
}

/// Get a calendar timestamp, in seconds, relative to UTC 1970-01-01.
/// Precision of timing depends on the hardware and operating system.
/// The return value is signed because it is possible to have a date that is
/// before the epoch.
/// See `std.os.clock_gettime` for a POSIX timestamp.
pub fn timestamp() i64 {
    return @divFloor(milliTimestamp(), ms_per_s);
}

/// Get a calendar timestamp, in milliseconds, relative to UTC 1970-01-01.
/// Precision of timing depends on the hardware and operating system.
/// The return value is signed because it is possible to have a date that is
/// before the epoch.
/// See `std.os.clock_gettime` for a POSIX timestamp.
pub fn milliTimestamp() i64 {
    return @as(i64, @intCast(@divFloor(nanoTimestamp(), ns_per_ms)));
}

/// Get a calendar timestamp, in microseconds, relative to UTC 1970-01-01.
/// Precision of timing depends on the hardware and operating system.
/// The return value is signed because it is possible to have a date that is
/// before the epoch.
/// See `std.os.clock_gettime` for a POSIX timestamp.
pub fn microTimestamp() i64 {
    return @as(i64, @intCast(@divFloor(nanoTimestamp(), ns_per_us)));
}

/// Get a calendar timestamp, in nanoseconds, relative to UTC 1970-01-01.
/// Precision of timing depends on the hardware and operating system.
/// On Windows this has a maximum granularity of 100 nanoseconds.
/// The return value is signed because it is possible to have a date that is
/// before the epoch.
/// See `std.os.clock_gettime` for a POSIX timestamp.
pub fn nanoTimestamp() i128 {
    if (builtin.os.tag == .windows) {
        // FileTime has a granularity of 100 nanoseconds and uses the NTFS/Windows epoch,
        // which is 1601-01-01.
        const epoch_adj = epoch.windows * (ns_per_s / 100);
        var ft: os.windows.FILETIME = undefined;
        os.windows.kernel32.GetSystemTimeAsFileTime(&ft);
        const ft64 = (@as(u64, ft.dwHighDateTime) << 32) | ft.dwLowDateTime;
        return @as(i128, @as(i64, @bitCast(ft64)) + epoch_adj) * 100;
    }

    if (builtin.os.tag == .wasi and !builtin.link_libc) {
        var ns: os.wasi.timestamp_t = undefined;
        const err = os.wasi.clock_time_get(os.wasi.CLOCK.REALTIME, 1, &ns);
        assert(err == .SUCCESS);
        return ns;
    }

    var ts: os.timespec = undefined;
    os.clock_gettime(os.CLOCK.REALTIME, &ts) catch |err| switch (err) {
        error.UnsupportedClock, error.Unexpected => return 0, // "Precision of timing depends on hardware and OS".
    };
    return (@as(i128, ts.tv_sec) * ns_per_s) + ts.tv_nsec;
}

test "timestamp" {
    const margin = ns_per_ms * 50;

    const time_0 = milliTimestamp();
    sleep(ns_per_ms);
    const time_1 = milliTimestamp();
    const interval = time_1 - time_0;
    try testing.expect(interval > 0);
    // Tests should not depend on timings: skip test if outside margin.
    if (!(interval < margin)) return error.SkipZigTest;
}

// Divisions of a nanosecond.
pub const ns_per_us = 1000;
pub const ns_per_ms = 1000 * ns_per_us;
pub const ns_per_s = 1000 * ns_per_ms;
pub const ns_per_min = 60 * ns_per_s;
pub const ns_per_hour = 60 * ns_per_min;
pub const ns_per_day = 24 * ns_per_hour;
pub const ns_per_week = 7 * ns_per_day;

// Divisions of a microsecond.
pub const us_per_ms = 1000;
pub const us_per_s = 1000 * us_per_ms;
pub const us_per_min = 60 * us_per_s;
pub const us_per_hour = 60 * us_per_min;
pub const us_per_day = 24 * us_per_hour;
pub const us_per_week = 7 * us_per_day;

// Divisions of a millisecond.
pub const ms_per_s = 1000;
pub const ms_per_min = 60 * ms_per_s;
pub const ms_per_hour = 60 * ms_per_min;
pub const ms_per_day = 24 * ms_per_hour;
pub const ms_per_week = 7 * ms_per_day;

// Divisions of a second.
pub const s_per_min = 60;
pub const s_per_hour = s_per_min * 60;
pub const s_per_day = s_per_hour * 24;
pub const s_per_week = s_per_day * 7;

/// An Instant represents a timestamp with respect to the currently
/// executing program that ticks during suspend and can be used to
/// record elapsed time unlike `nanoTimestamp`.
///
/// It tries to sample the system's fastest and most precise timer available.
/// It also tries to be monotonic, but this is not a guarantee due to OS/hardware bugs.
/// If you need monotonic readings for elapsed time, consider `Timer` instead.
pub const Instant = struct {
    timestamp: if (is_posix) os.timespec else u64,

    // true if we should use clock_gettime()
    const is_posix = switch (builtin.os.tag) {
        .wasi => builtin.link_libc,
        .windows => false,
        else => true,
    };

    /// Queries the system for the current moment of time as an Instant.
    /// This is not guaranteed to be monotonic or steadily increasing, but for most implementations it is.
    /// Returns `error.Unsupported` when a suitable clock is not detected.
    pub fn now() error{Unsupported}!Instant {
        // QPC on windows doesn't fail on >= XP/2000 and includes time suspended.
        if (builtin.os.tag == .windows) {
            return Instant{ .timestamp = os.windows.QueryPerformanceCounter() };
        }

        // On WASI without libc, use clock_time_get directly.
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var ns: os.wasi.timestamp_t = undefined;
            const rc = os.wasi.clock_time_get(os.wasi.CLOCK.MONOTONIC, 1, &ns);
            if (rc != .SUCCESS) return error.Unsupported;
            return Instant{ .timestamp = ns };
        }

        // On darwin, use UPTIME_RAW instead of MONOTONIC as it ticks while suspended.
        // On linux, use BOOTTIME instead of MONOTONIC as it ticks while suspended.
        // On freebsd derivatives, use MONOTONIC_FAST as currently there's no precision tradeoff.
        // On other posix systems, MONOTONIC is generally the fastest and ticks while suspended.
        const clock_id = switch (builtin.os.tag) {
            .macos, .ios, .tvos, .watchos => os.CLOCK.UPTIME_RAW,
            .freebsd, .dragonfly => os.CLOCK.MONOTONIC_FAST,
            .linux => os.CLOCK.BOOTTIME,
            else => os.CLOCK.MONOTONIC,
        };

        var ts: os.timespec = undefined;
        os.clock_gettime(clock_id, &ts) catch return error.Unsupported;
        return Instant{ .timestamp = ts };
    }

    /// Quickly compares two instances between each other.
    pub fn order(self: Instant, other: Instant) std.math.Order {
        // windows and wasi timestamps are in u64 which is easily comparible
        if (!is_posix) {
            return std.math.order(self.timestamp, other.timestamp);
        }

        var ord = std.math.order(self.timestamp.tv_sec, other.timestamp.tv_sec);
        if (ord == .eq) {
            ord = std.math.order(self.timestamp.tv_nsec, other.timestamp.tv_nsec);
        }
        return ord;
    }

    /// Returns elapsed time in nanoseconds since the `earlier` Instant.
    /// This assumes that the `earlier` Instant represents a moment in time before or equal to `self`.
    /// This also assumes that the time that has passed between both Instants fits inside a u64 (~585 yrs).
    pub fn since(self: Instant, earlier: Instant) u64 {
        if (builtin.os.tag == .windows) {
            // We don't need to cache QPF as it's internally just a memory read to KUSER_SHARED_DATA
            // (a read-only page of info updated and mapped by the kernel to all processes):
            // https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntddk/ns-ntddk-kuser_shared_data
            // https://www.geoffchappell.com/studies/windows/km/ntoskrnl/inc/api/ntexapi_x/kuser_shared_data/index.htm
            const qpc = self.timestamp - earlier.timestamp;
            const qpf = os.windows.QueryPerformanceFrequency();

            // 10Mhz (1 qpc tick every 100ns) is a common enough QPF value that we can optimize on it.
            // https://github.com/microsoft/STL/blob/785143a0c73f030238ef618890fd4d6ae2b3a3a0/stl/inc/chrono#L694-L701
            const common_qpf = 10_000_000;
            if (qpf == common_qpf) {
                return qpc * (ns_per_s / common_qpf);
            }

            // Convert to ns using fixed point.
            const scale = @as(u64, std.time.ns_per_s << 32) / @as(u32, @intCast(qpf));
            const result = (@as(u96, qpc) * scale) >> 32;
            return @as(u64, @truncate(result));
        }

        // WASI timestamps are directly in nanoseconds
        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            return self.timestamp - earlier.timestamp;
        }

        // Convert timespec diff to ns
        const seconds = @as(u64, @intCast(self.timestamp.tv_sec - earlier.timestamp.tv_sec));
        const elapsed = (seconds * ns_per_s) + @as(u32, @intCast(self.timestamp.tv_nsec));
        return elapsed - @as(u32, @intCast(earlier.timestamp.tv_nsec));
    }
};

/// A monotonic, high performance timer.
///
/// Timer.start() is used to initialize the timer
/// and gives the caller an opportunity to check for the existence of a supported clock.
/// Once a supported clock is discovered,
/// it is assumed that it will be available for the duration of the Timer's use.
///
/// Monotonicity is ensured by saturating on the most previous sample.
/// This means that while timings reported are monotonic,
/// they're not guaranteed to tick at a steady rate as this is up to the underlying system.
pub const Timer = struct {
    started: Instant,
    previous: Instant,

    pub const Error = error{TimerUnsupported};

    /// Initialize the timer by querying for a supported clock.
    /// Returns `error.TimerUnsupported` when such a clock is unavailable.
    /// This should only fail in hostile environments such as linux seccomp misuse.
    pub fn start() Error!Timer {
        const current = Instant.now() catch return error.TimerUnsupported;
        return Timer{ .started = current, .previous = current };
    }

    /// Reads the timer value since start or the last reset in nanoseconds.
    pub fn read(self: *Timer) u64 {
        const current = self.sample();
        return current.since(self.started);
    }

    /// Resets the timer value to 0/now.
    pub fn reset(self: *Timer) void {
        const current = self.sample();
        self.started = current;
    }

    /// Returns the current value of the timer in nanoseconds, then resets it.
    pub fn lap(self: *Timer) u64 {
        const current = self.sample();
        defer self.started = current;
        return current.since(self.started);
    }

    /// Returns an Instant sampled at the callsite that is
    /// guaranteed to be monotonic with respect to the timer's starting point.
    fn sample(self: *Timer) Instant {
        const current = Instant.now() catch unreachable;
        if (current.order(self.previous) == .gt) {
            self.previous = current;
        }
        return self.previous;
    }
};

test "Timer + Instant" {
    const margin = ns_per_ms * 150;

    var timer = try Timer.start();
    sleep(10 * ns_per_ms);
    const time_0 = timer.read();
    try testing.expect(time_0 > 0);
    // Tests should not depend on timings: skip test if outside margin.
    if (!(time_0 < margin)) return error.SkipZigTest;

    const time_1 = timer.lap();
    try testing.expect(time_1 >= time_0);

    timer.reset();
    try testing.expect(timer.read() < time_1);
}

test {
    _ = epoch;
}